Process for producing elevated temperature corrosion resistant metal articles

ABSTRACT

A process for providing coatings on metal articles whereby the articles will be resistant to corrosion at elevated temperatures. The process involves the application of an overlay on an article surface, the overlay comprising a ductile metal of a composition normally resistant to corrosion at elevated temperatures. An outer layer of aluminide or metal which is resistant to corrosion at elevated temperatures but which is subject to embrittlement at such temperatures is applied to complete the coating. Porosity in the coating is then eliminated and a high integrity corrosion resistant coating not subject to cracking is obtained by heating the article in a gaseous atmosphere to elevated temperature and simultaneously applying isostatic pressure to the article.

This invention relates to metal articles which are subjected to elevatedtemperatures during use. In particular, the invention is concerned witha process for significantly improving the corrosion resistance of sucharticles under such conditions whereby more satisfactory performance andlonger life for such articles can be obtained.

There are many applications which involved elevated temperature exposureof metal components. Such applications involve, for example, variousaerospace applications and land operations such as components utilizedin gas turbine engines.

In all such applications, it is important to provide some means forpreventing undue corrosion of the components involved since suchcorrosion will materially shorten the useful life of the components, andcan create significant performance and safety problems. Various alloysincluding most superalloys are characterized by a degree of corrosionresistance; however, such resistance is significantly decreased whenunprotected superalloy components are exposed at the operatingtemperatures involved in certain systems. For that reason, suchcomponents have been provided with coatings, such as aluminide coatings,which increase the corrosion resistance at the extreme operatingtemperatures.

Aluminide coatings are applied by pack cementation process. In thisprocess, the substrate chemistry and the processing temperature exert amajor influence on coating chemistry, thickness and properties.Specifically, the coatings comprise a hard, brittle outer layer and ahard, brittle multi-phase sublayer that can crack when subjected tooperating conditions. This leads to poor fatigue properties and thecracks also materially reduce the corrosion resistance of the coatedcomponents.

Another class of coatings is the MCrAlY overlay coatings where M standsfor a transition metal element such as Fe, Co, or Ni. Presently, thesecoatings are applied by vacuum vapor deposition of MCrAlY alloy on asuperalloy surface. Such vapor coatings have been shown to have certainadvantages over aluminide coatings in providing extended life to turbinecomponents. Unfortunately, such coatings may contain radially orienteddefects which are created during the vapor deposition processing. Suchdefects are the sites of corrosion attack at high temperature which canlead to premature failure of the coated part. Further, the vaporcoatings are relatively costly to produce and require relativelyexpensive manufacturing equipment.

In the past, several low cost methods such as plasma spraying, slurryingsintering, etc. have been investigated to process MCrAlY coatings onsuperalloys. However, most of these attempts have resulted inapplication of a porous coating which prematurely fails due to corrosionattack.

It is an object of this invention to provide metal articles which areparticularly capable of resisting corrosion under elevated temperatureoperating conditions.

It is a more specific object of this invention to provide and improvedprocess for the treating of superalloys and other metals exposed toelevated temperature operations whereby such articles will resistcorrosion under such conditions.

It is also an object of this invention to provide a process for coatingmetal articles whereby coatings which are highly resistant to corrosionat elevated temperatures can be utilized without embrittlement orcracking so that the physical properties of the articles and thecorrosion resistance thereof remain at high levels during use of thearticles.

A more specific object of the invention is to provide a high integrityplasma sprayed metallic coating for enhanced corrosion protection andductility.

These and other objects of this invention will appear hereinafter andfor purposes of illustration, but not of limitation, the accompanyingdrawing illustrates as follows:

FIG. 1 is a chart comparing the various coatings for the nickel basesuperalloys in terms of durability at 1750° F. peak temperature;

FIG. 2 is a photomicrograph at 500X of the coating matrix as plasmasprayed during the method of the present invention;

FIG. 3 is a photomicrograph at 500X of the coating matrix as plasmasprayed and aluminized and hot isostatically pressed during the methodof the present invention;

FIG. 4 is an electron microprobe trace depicting the Al, Co, Cr and Nicontent in a plasma sprayed CoCrAlY coating according to the presentinvention; and,

FIG. 5 is an electron microprobe trace depicting the Al, Co, Cr and Nicontent in a plasma sprayed and aluminized and hot isostatically pressedCoCrAlY coating according to the present invention.

This invention generally involves a process for producing a coating onmetallic articles for purposes of rendering the articles resistant tocorrosion at elevated temperatures. The process first involves theapplication of a ductile metallic overlay on the article surface. Theoverlay is of a composition normally resistant to corrosion at elevatedtemperatures.

An outer layer is applied over the overlay, the outer layer also beingformed of a material resistant to corrosion at elevated temperatures.The process thereafter involves subjecting the articles to a hotisostatic pressing operation wherein the article is simultaneouslysubjected to high temperature and high pressure applied through agaseous atmosphere. The temperature and pressure relationship is suchthat porosity in the coating composed of the overlay and outer layerwill be eliminated and the chemical composition of the coating will bemodified by inter-diffusion between the overlay, the outer layer and thesubstrate at such temperatures. The resultant high integrity coatingwill be suitable for performance under elevated temperature conditionsbeing developed.

The ductile metallic overlay which is applied directly to the articlesurface preferably comprises an alloy having as a base element atransition metal comprising cobalt, iron or nickel. Amounts of chromium,aluminum and/or yttrium are alloyed with the base metal pursuant to thepreferred practice of the invention.

The ductile metallic layer can be conveniently applied by plasmaspraying or by other conventional means such as pressure bonding,physical vapor deposition, sputtering, ion plating, and slurryingsintering. Where plasma spraying is employed, the overlay material isheated to a highly plastic, or molten state such that wetting ordeformation interlocking of the particles being deposited isaccomplished as the particles strike the substrate surface. Plasmaspraying is particularly desirable since it is a generally less costlytechnique for accomplishing the overlay coating, and since the techniqueis applicable to all contemplated coating compositions.

The metallic overlay which is thus achieved will, because of thecomposition of the coating, generally improve the elevated temperaturecorrosion resistance of the article coated, however, the coatings arecharacterized by a degree or porosity which adversely affects suchelevated temperature characteristics.

As indicated, this invention involves the application of an outer layerto the overlay. This outer layer also comprises a material which isresistant to corrosion at elevated temperatures. This material, like theaforementioned overlay, suffers from certain deficiencies from thestandpoint of elevated temperature corrosion resistance if used as theonly coating on the article involved. Aluminide coatings represent onetype of outer layer contemplated, and such coatings, when applieddirectly to a substrate, have a tendency to become embrittled and/or todevelop cracks whereby the utility of such coatings for protectionagainst corrosion is minimized.

In addition to aluminide coatings, the invention contemplates otherlayers such as precious metals and their alloys, these metals/alloysalso being used in combination with the overlay described. Thecombination eliminates the deficiencies which are found when either theoverlay materials or the outer layer materials are used alone inconjunction with a given substrate. This elimination of deficienciesoccurs, in particular and in accordance with this invention, when thearticles having the overlay and outer layer are hot isostaticallypressed. Gold, palladium, platinum and rhodium are contemplated asprecious metals suited for the practice of the invention.

In the case of aluminide coatings, the outer layer may be applied bypack cementation or other conventional techniques such as dipping,spraying, metallizing and electrophoresis. Where precious metals areused for purposes of forming the outer layer, conventional techniquessuch as plasma spraying, ion plating, electron beam or vapor deposition,sputtering, slurry sintering or pressure bonding may be utilized.

The conditions for hot isostatic pressing contemplated in accordancewith this invention may be determined by reference to the conditionsrecommended for the substrate. Thus, hot isostatic pressing techniquesare recommended for superalloys and other materials utilized forelevated temperature applications, particularly for purposes ofeliminating defects which develop during casting. Generally, suchtechniques involve the application of pressure through a gaseousatmosphere in the order of 10,000 to 50,000 psi. The temperature in theautoclave employed for the hot isostatic pressing will generally be in arange of 50° below the gamma prime solvus temperature of the castings upto the solidus temperature of the castings.

Where aluminide outer layers are utilized, the presence of aluminiumunder the conditions of hot isostatic pressing leads to the enrichmentof the underlying coating. In addition, a selective outward diffusion ofbase substrate element, such as nickel in the case of nickel base alloysubstrates, occurs into the coating during hot isostatic pressing. Thisdiffusion modifies the chemical composition of the MCrAlY overlayaluminide outer layer. Thus, a failsafe system is provided. Thealuminide layer has a lessened tendency to crack because it is supportedby a ductile and sound (defect free) layer, not a brittle multiphaselayer that is conventionally the case. If a crack occurs in thealuminide outer layer, the ductility of the overlay restricts itspropagation. Widespread oxidation of the overlay does not occur becausethe completely dense and chemically modified MCrAlY overlay isoxidation/corrosion resistant.

Where precious metals are employed, the advantages referred to are alsoavailable. Thus, any tendency of such metals to embrittle or crack whenapplied directly to a substrate is eliminated by interposing the overlaycoating and as a result of the subsequent hot isostatic pressing.

The application of the two layers has the further advantage of servingto encapsulate the article involved whereby surface connected defects inthe article will not be exposed to the high pressure atmosphere duringhot isostatic pressing. The coatings thereby function as a means forachieving elimination of such surface connected defects since, as setforth in prior teachings, the temperature and pressure conditions of thehot isostatic pressing will result in metal movement to the extent thatsuch defects are eliminated.

The coatings referred to herein, when subjected to the hot isostaticpressing, are characterized by elevated temperature fatigue resistanceand ductility in addition to the corrosion resistance referred to. Thisconstitutes a necessary feature of such coatings in view of theapplication involved. Thus, the nickel base and cobalt base superalloysas well as dispersion strengthened alloys, composites, and directionaleutectics which are contemplated for treatment in accordance with thisinvention are employed in applications where fatigue resistance andductility at elevated temperatures are critical factors.

As noted, the optimum overlay composition comprise a cobalt, iron ornickel base material with aluminum, yttrium and chromium additions. Thealuminum values, whether initially included in the overlay or obtainedfrom an aluminide outer layer, provides for Al₂ O₃ formation with theattendant oxidation resistance. Yttrium and equivalent additions achievethe promotion of oxide adherents and the chromium values enhance the Al₂O₃ formation while also providing hot corrosion resistance.

Aluminide coatings when utilized alone will not consistently exhibitlong-time oxidation, sulfidation and thermal fatigue resistance. Thesecoatings typically contain continuous phases of limited ductility whichtend to crack under high corrosive stresses. Once cracks develop, anoxidizing or other hot corrosive atmosphere can gain access to theunderlying substrate. As indicated, the presence of the intermediateoverlay coating, in combination with the hot isostatic pressing, avoidssuch problems. Thus, the advantages of an aluminide layer without thedifficulties previously experienced can be obtained.

The utilization of the overlay coating also enables the efficientintroduction of elements such as yttrium which have been difficult toincorporate in nickel aluminide coatings. Such elements are alreadyincorporated in the overlay, and in addition, broader ranges of nickeland aluminum compositions in the aluminide layer can be achieved when anoverlay is utilized whereby prior limitations on mechanical propertiesof the aluminide coatings can be avoided.

The following comprises an example of the practice of this invention.

EXAMPLE I

A typical nickel base superalloy of the type used in gas turbine engineswas coated with CoCrAlY overlay. The superalloy, known as IN792+Hf, hada nominal composition of 0.15% C, 12.22% Cr, 9.04% Co, 1.97% Mo, 3.97%W. 3.92% Ta, 3.88% Ti, 3.57% Al, 0.85% Hf, 0.017% B, 0.10% Zr andbalance nickel. The nominal composition of overlay was, by weightpercent 23 Cr, 13 Al, 0.6 Y and the balance cobalt, and this coating wasapplied by a plasma spray process. The coating powder was sprayed usinga high velocity gun (Mach 3) operating at 76 kw with argon and helium asprimary and secondary gases, respectively. Spraying was performed in achamber maintained at a pressure of 50 torr. The plasma spray parametersare summarized below:

    ______________________________________                                        Gun to workpiece distance                                                                            16 in.                                                 Primary gas (argon) V  600 CFH                                                P                      250 psi                                                Secondary gas (helium) V                                                                             150 CFH                                                P                      250 psi                                                Voltage                85 volts                                               Current                900 amps                                               Powder Flow            0.1 lb. PM                                             Carrier gas (argon)    50 CFH                                                 ______________________________________                                    

The overlay coating was aluminized by the pack cementation method. Thismethod is described in Freeman, et al. U.S. Pat. No. 3,625,750 issued onDec. 7, 1971. The source of aluminum as a powder mixture consisting of35% aluminium oxide, 67% chromium/aluminum alloy and 0.02% to 0.05%ammonium chloride. The process is conducted at 1900° F. to 1950° F. in areduced pressure atmosphere. The aluminized-overlay coating thusobtained was hot isostatically pressed at 2200° F. and 15 ksi pressurefor two hours in argon atmosphere.

A 500X photomicrograph of the plasma sprayed CoCrAlY overlay coating inthe unetched condition is shown in FIG. 2. A high degree (5% by volume)of porosity is visible in the coatint which is an intimate mixture ofCoAl (β) and Co-solid solution (γ) phases. FIG. 3 depicts the 500Xphotomicrograph of the coating which has been plasma sprayed,aluminized, and hot isostatically pressed. The coating contains noporosity. Examinations were also made of articles which were providedwith plasma sprayed and hot isostatically pressed CoCrAlY coating inwhich a fair amount of porosity was observed. Where an aluminide coatingwas provided as an outer layer over a CoCrAlY overlay, no porosity wasobserved indicating that the hot isostatic pressing was effective toeliminate the porosity only after application of the aluminide coating.

Another microstructural change which occurs when the plasma sprayedcoating is subjected to an aluminizing and hot isostatic pressureoperation is the modification of the chemical composition of thecoating. FIGS. 4 and 5 represent the electron microprobe traces(chemical composition) of Al, Co, Cr and Ni elements for an IN792+Hfsubstrate after plasma spraying (FIG. 4), and after plasma spraying,aluminizing, and hot isostatic pressng (FIG. 5). As can be noted fromthese traces, due to the aluminizing and HIPing operations, aconcentration gradient of aluminum ranging between about 35 weightpercent at the outer edge of the coating to about 5 weight percent atthe coating-substrate interface is developed. Also, extensive amounts ofnickel ranging between 10 weight percent at the outer edge of thecoating and 40 weight percent at the coating-substrate interface hasdiffused inside the coating. This diffusion of aluminum and nickel hasmodified the concentration of chromium and cobalt elements in accordancewith the thermodynamic stability of (Co,Ni)Al and (Co,Ni) solid solutionphases. Thus, extensive modification of the chemical composition of theplasma sprayed CoCrAlY coating takes place after aluminizing and HIPingprocesses.

The performance of articles coated pursuant to this invention wasevalulated by using a 0.7 Mach burner rig testing. The testing cycle was1750° F./2 minutes; 1450° F./4 minutes; 1750° F./2 minutes; air cool/2minutes with 5 ppm salt injection into a flame containing 0.2% sulphur.Such testing highlights the sulfidation phenomena and imposessignificant thermal stresses on the protection system and the surfaceoxide.

A comparative graph representing the life of various coatings subjectedto above described test conditions is given in FIG. 1. The articlescoated in accordance with this invention demonstrated a burner rig lifeabout five times more than a typical aluminide coating and about one anda half to two times greater than lives exhibited by the overlay coatingsprocessed by physical vapor deposition or plasma spray processes.

As indicated, the substantial increase in coating life is attributed tothe presence of a large reservoir of aluminum, (Co,Ni)Al phase, in theouter layer of the coating for superior oxidation/corrosion resistance.This layer is supported by a ductile (Co, Ni) solid solution layerthereby providing superior resistance to thermal fatigue. In addition,absence of any defects (porosity) in the coating has left no shortcircuit paths for corrosion attack to follow; thus increasing theprotective capability of the coating in comparison to as plasma sprayedor as plasma sprayed and aluminized CoCrAlY coatings.

Essentially corresponding procedures can be followed with other knowncoating compositions, for examle, alloys consisting essentially of 15-40weight percent chromium, 10-25 weight percent aluminum, 0.01 to 5 weightpercent of a member selected from the group consisting of the rareearths and yttrium, and the balance iron, cobalt or nickel. Examples ofother coating materials and coating processes are found in U.S. Pat.Nos. 3,676,085, 3,754,903, 3,873,347, 3,928,026 and 3,961,098.

It will be understood that various changes and modifications may be madein the above described invention which provide the characteristics ofthis invention without departing from the spirit thereof particularly asdefined in the following claims.

That which is claimed is:
 1. In a process for providing a coating on ametallic substrate, the coating renderng the substrate resistent tocorrosion at elevated temperatures, said process comprising the steps ofproviding a ductile metallic overlay on the substrate surface, saidoverlay being of a composition normally resistant to corrosion atelevated temperatures, and applying an outer layer on said overlay, saidouter layer being formed of a material more resistant to corrosion atelevated temperatures, the outer layer comprising an encapsulating meansthereby preventing penetration of gas, the improvement comprising thesteps of thereafter subjecting said coated substrate to a hot isostaticpressing operation by locating the coated substrate in a pressure-tightchamber, and subjecting the coated substrate to a temperature andpressure applied through a gaseous atmosphere sufficient to eliminateporosity, said temperature and pressure application also causingdiffusion of substrate ingredients from one direction into the overlayand diffusion of outer layer ingredients from the other direction intothe overlay thereby modifying the composition of the coating composed ofsaid overlay and said outer layer.
 2. A process in accordance with claim1 wherein said outer layer comprises an aluminide coating.
 3. A processin accordance with claim 1 wherein said overlay comprises an alloyhaving as a base constituent at least one of the elements selected fromthe group consisting of iron, cobalt and nickel. .[.4. A process inaccordance with claim 3 wherein said overlay is applied to said articlesurface by one of the methods selected from the group consisting ofplasma spraying, pressure bonding, electron beam or vapor deposition,sputtering, ion plating and slurry sintering..]. .[.5. A process inaccordance with claim 3 wherein said aluminide coating is applied by oneof the methods selected from the group consisting of pack cementation,dipping, spraying, metallizing, and electrophoresis..].
 6. An articleproduced in accordance with the process of claim
 1. 7. A process inaccordance with claim 1 wherein the base metal of the substrate alloy isselected from the group consisting of Ni, Co and Fe. .[.8. A process inaccordance with claim 3 wherein said overlay is applied to said articlesurface by one of the methods selected from the group consisting ofplasma spraying and slurry sintering..]. .Iadd.
 9. A process inaccordance with claim 2 wherein said aluminide coating is applied bypack cementation, dipping, spraying, metallizing, or electrophoresis..Iaddend. .Iadd.
 10. A process in accordance with claim 3 wherein saidoverlay is applied to said article surface by plasma spraying, pressurebonding, electron beam or vapor deposition, sputtering, ion plating orslurry sintering. .Iaddend..Iadd.
 11. A process in accordance with claim3 wherein said overlay is applied to said article surface by plasmaspraying or slurry sintering. .Iaddend. .Iadd.
 12. A method for coatinga superalloy substrate with an oxidation-corrosion protective MCrAlYtype coating where M is selected from the group consisting of nickel,cobalt and iron, comprising the steps of:(a) plasma spraying the MCrAlYcoating onto the superalloy substrate, the coating being characterizedas having pores, voids and similar defects, some of which extend to thefree surface of the coating, said defects reducing the protectiveness ofthe coating: (b) sealing the free surface of the MCrAlY coating byproviding a metallic envelope thereover, said envelope spanning andsealing the defects which extend to the free surface of the coating (c)hot isostatically pressing the coated substrate at a sufficient pressureand temperature and for a sufficient time to close the defects internalof the MCrAlY coating and those intersecting said free surface and todiffuse at least a portion of the metallic envelope into the MCrAlYcoating, closure of said defects and diffusion of said metal envelopeinto the coating significantly enhancing the oxidation-corrosionprotective properties of the coating. .Iaddend..Iadd.
 13. The method ofclaim 12 wherein the metallic coating is aluminum. .Iaddend..Iadd. 14.The method of claim 12 wherein the metallic envelope is provided byelectroplating the free surface to deposit a metallic coating thereon..Iaddend.